The New Idria serpentinite body in the Coast Ranges of California is a diapir that resulted from the interaction of the migrating Mendocino trench-ridge-transform fault triple junction, transpression, metasomatic fluids, and previously subducted oceanic crust and mantle. Northward propagation of the San Andreas fault progressively eliminated the original subduction zone, allowing seawater to penetrate into the formerly subducting abyssal peridotite mantle, triggering serpentinization. The associated physical changes in density, volume, and strength yielded an expanding, buoyantly rising serpentinite protrusion, facilitated by transpression along the San Andreas fault. Sedimentary facies and intrusion of minor cross cutting syenite and alkali basalt dikes indicate that the serpentinization-driven diapir buoyantly rose and widely breached the surface by ca. 14 Ma, attending migration of the Mendocino Triple Junction past the latitude of New Idria.

Introduction: Diamond of metamorphic origin in supracrustal rocks was first discovered from the Kokchetav Massif in the Caledonides of northern Kazakhstan. Microdiamond-bearing rocks include biotite gneiss, garnet mica schist, pyroxene marble, and garnet pyroxenite; no diamond has been reported in the associated metabasaltic eclogitic rocks (e.g., Sobolev & Shatsky, 1990). We have selected 3 eclogites, 5 garnet-bearing gneisses and 2 pyroxene marbles collected from Kumdikol of the northern Kokchetav Massif during the 1994 field season for detailed study for two purposes: (1) to determine whether or not the microdiamonds were formed within the diamond stability field; and (2) to delineate the retrogressive P-T path.

Naturally occurring Cr(VI) has been ascribed to terrestrial Cr(III) oxidation by Mn (di)oxides, generated through reaction of Mn(II) with molecular oxygen (O-2). However, hydrogen peroxide (H2O2) is a potential oxidant of Cr(III) that may form in serpentinization (high H-2, low O-2) systems where chromite [i.e., the main mineralogical source of Cr(III)] is abundant. Accordingly, here we evaluate H2O2 and chromite interactions in serpentinization systems to determine pathways of Cr(III) oxidation that alters the current paradigm of O-2-dependent oxidation. Field observations support that metastable H2O2 and Cr(VI) are present in serpentinization-related fluids relatively absent of O-2. Further, laboratory experiments demonstrate and support that H2O2 is a kinetically facile oxidant of chromite, especially under alkaline conditions, which provides a variety of alternative means by which Cr(VI) may be generated and supplied to the oceans not directly linked to atmospheric O-2. Thus, Cr(III) oxidation pathways, and their influence on the Cr isotopic record, must account for anoxic Cr(III) oxidation in serpentinization systems as well as a variety of H2O2-induced Cr(III) oxidation pathways that may occur in both terrestrial and marine systems.

Initiation of sea floor spreading in the Red Sea 4-5 m.y. ago was preceded by an early Tertiary period of continental extension accompanied by intrusive and extrusive magmatic activity. During the Miocene, tholeiitic basaltic magma was intruded along faults and fractures in rifted continental crust to form extensive dike swarms and layered gabbroic plutons along with associated silicic intrusive bodies. These intrusions, best exposed sporadically in a 4-6 km wide zone at the edge of the Red Sea coastal plain in southwest Saudi Arabia, are known collectively as the Tihama Asir complex. One of these intrusions, a layered gabbro at Jabal Tirf near Jizan, consists of clinopyroxene gabbro and clinopyroxene olivine gabbro, plus minor troctolite and anorthosite, displaying cumulate textures. Microprobe analyses performed on a sequence of samples, from the bottom to the top of the intrusion, reveal no systematic variation in mineral chemistry with stratigraphic height in the intrusion. Mineral compositions in the Jabal Tirf layered gabbro are similar to the Skaergaard Lower Zone a (LZa), but the intrusion shows no iron-enrichment trend like the Skaergaard. The mineral chemistry of the Jabal Tirf layered gabbro suggests crystallization at approximately 1 kb, liquidus temperature about 1200°C, solidus temperature about 950°C, and low magma water content. The layered gabbro formed by multiple influxes of already differentiated tholeiitic magma from a single source, possibly a lower crustal magma chamber. Associated diabase dikes formed from the same parental magma as the gabbro. Cogenetic silicic intrusives may result from differentiation of basaltic magma and partial melting of lower continental crust.

In 1949, little did I realize, as I began my PhD studies at Stanford University on the mineralogy of the New Idria serpentinite body in the Diablo Range of central California, that the ideas in my thesis (Coleman 1957) would become anachronistic. At the time I began my field work in New ldria, the Johns Manville and Union Carbide companies had begun open pit mining for short-fiber asbestos. Their operations exposed numerous tectonic inclusions of blueschist metamorphic rocks containing jadeite and glaucophane as well as small ophiolitic fragments. With these new exposures it became obvious that the New Idria serpentinite massif was not an igneous intrusion but instead consisted mostly of highly sheared melange within an elongate, diapiric dome protruding into an anticlinal fold in Mesozoic sediments! Today, this famous serpentinite massif is better described as the end product of tectonically destroyed ophiolite.

Serpentine soils derived from the weathering of ultramafic rocks and their metamorphic derivatives (serpentinites) are chemically prohibitive for vegetative growth. Evaluating how serpentine vegetation is able to persist under these chemical conditions is difficult to ascertain due to the numerous factors (climate, relief, time, water availability, etc.) controlling and affecting plant growth. Here, the uptake, incorporation, and distribution of a wide variety of elements into the biomass of serpentine vegetation has been investigated relative to vegetation growing on an adjacent chert-derived soil. Soil pH, electrical conductivity, organic C, total N, soil extractable elements, total soil elemental compositions and plant digestions in conjunction with spider diagrams are utilized to determine the chemical relationships of these soil and plant systems. Plant available Mg and Ca in serpentine soils exceed values assessed in chert soils. Magnesium is nearly 3 times more abundant than Ca in the serpentine soils; however, the serpentine soils are not Ca deficient with Ca concentrations as high as 2235mgkg−1. Calcium to Mg ratios (Ca:Mg) in both serpentine and chert vegetation are greater than one in both below and above ground tissues. Soil and plant chemistry analyses support that Ca is not a limiting factor for plant growth and that serpentine vegetation is actively moderating Mg uptake as well as tolerating elevated concentrations of bioavailable Mg. Additionally, results demonstrate that serpentine vegetation suppresses the uptake of Fe, Cr, Ni, Mn and Co into its biomass. The suppressed uptake of these metals mainly occurs in the plants’ roots as evident by the comparatively lower metal concentrations present in above ground tissues (twigs, leaves and shoots). This research supports earlier studies that have suggested that ion uptake discrimination and ion suppression in the roots are major mechanisms for serpentine vegetation to tolerate the chemistry of serpentine soils.

Metamorphic hydration and oxidation of ultramafic rocks produces serpentinites, composed of serpentine group minerals and varying amounts of brucite, magnetite, and/or FeNi alloys. These minerals buffer metamorphic fluids to extremely reducing conditions that are capable of producing hydrogen gas. Awaruite, FeNi3, forms early in this process when the serpentinite minerals are Fe-rich. Olivine with the current mantle Fe/Mg ratio was oxidized during serpentinization after the Moon-forming impact. This process formed some of the ferric iron in the Earth's mantle. For the rest of Earth's history, serpentinites covered only a small fraction of the Earth's surface but were an important prebiotic and biotic environment. Extant methanogens react H2with CO2to form methane. This is a likely habitable environment on large silicate planets. The catalytic properties of FeNi3allow complex organic compounds to form within serpentinite and, when mixed with atmospherically produced complex organic matter and waters that circulated through basalts, constitutes an attractive prebiotic substrate. Conversely, inorganic catalysis of methane by FeNi3competes with nascent and extant life.

Symplectite textures are common product of retrogression in the intermediate-to high-temperature metamorphic rocks such as granulite and eclogite, and contain important information on the P-T histories.Their formation mechanisms are generally attributed to the fast nucleation of symplectite minerals replacing reactant peak-stage mineral due to rapid changes in pressure and/or temperature.Alternatively, their textural characteristics are controlled by kinetics and dynamics of metamorphic recrystallization.Although an infinite number of symplectite variations have been known in many high-grade metamorphic rocks, here we present a rare example of coarsegrained symplectite in low-T jadeitite from the northern Motagua Fault Zone(MFZ)of Guatemala(Fig. 1) .This region is one of the few jadeitite localities in the world to have produced excellent quality of 'Jade'.As shown in figures, coarse-grained jadeitite(Fig.2)contains spectacular symplectitic intergrowth of albite(Ab) ＋ analcime(Anl) ± nepheline(Ne)that formed by jadeite breakdown(Figs.3 and4) .Textural relations and retrograde phase assemblage suggest that precursor jadeite has reacted with H2O-rich fluid to form symplectite under the approximate P-T condition at P< 0.7 GPa and T< 400 ℃.The development of symplectite may have been promoted by fluid infiltration most probably from serpentinite to jadeitite.

This short report attempts to illuminate the geological features that contributed to making the Jasper Ridge Biological Preserve (JRBP) a unique research island within the rapidly urbanized San Francisco Peninsula. Written in the rocks of the Preserve is a history of continental growth that extends back to the Jurassic (~150 Ma) as the California continental margin grew by subduction and accretion. Movement along the San Andreas fault system has left an indelible mark on the topography by uplift and faulting, and is even now changing the landscape by measurable increments. The sediments of Searsville Lake preserve a chronology of logging in the last century and housing development in more recent decades. Continued multidisciplinary study of this dynamic island of preserved interlocking biological, geological, and hydrogeological records will enhance student, faculty, and docent research, and our understanding of this complex area.

Serpentine soils derived from the weathering of ultramafic rocks, mainly ophiolitic serpentinites, are typically characterized by Cr concentrations in excess of 200 mg kg-1, comparatively higher than non-serpentine soils. We review the chemistry of Cr in serpentine soils and their protoliths, focusing on serpentine soils collected from New Caledonia, Oregon, and California. Overall, serpentine soils are slightly acidic (average pH of ∼6), contain a variety Fe(III) oxides (magnetite and hematite), Fe(III) (oxy)hydroxides, phyllosilicates (serpentine and chlorite), and clays (smectites and vermiculites), and contain concentrations of Cr (> 200 mg kg-1), Ni (> 1,000 mg kg-1), and Mn (> 200 mg kg-1) exceeding values of non-serpentine soils. Although Cr concentrations in serpentine soils have been reported as high as 6 wt% in New Caledonia, Cr values in New Caledonia, Oregon, and California serpentine soils evaluated in this study range from 827 to 9,528 mg kg-1. Chromium(III) is the only valence state observed in the serpentine soil solids; however, Cr(VI) has been identified in New Caledonia and California serpentine soil solutions at concentrations below 30 μM. The enrichment and range of Cr concentrations in serpentine soils are directly related to the presence of Cr-spinels, specifically chromite and Cr-magnetite. These phases are resistant to weathering and are preserved in the soil environment; however, oxidation of Cr(III) from Cr-spinels by high-valent Mn oxides, or other strong oxidants, is a potential source of Cr(VI) identified in serpentine soil solutions. Due to the weathering resistant nature of the Cr-spinels, Cr-bearing silicates including clay minerals, Cr-chlorite, Cr-garnet, Cr-mica, and Cr-epidote are more viable sources of Cr identified in vegetation, soil extractions, soil solutions, and related waters.

Based upon the P- and S-wave data acquired along the geoscience transect from Allay to Altun Tagh in Northwest, China, the crustal structures of velocities and Poisson's ratio are determined. The crustal velocity structure features an obvious three-layer structure with velocities of 6.0 similar to 6.3km/s 6.3 similar to 6.6km/s and 6.9 similar to 7.0km/s from surface to depth, respectively. The crustal thickness along the entire profile is mostly 50km with the thickest crust (56km) beneath the Altay and the thinnest (46km) beneath the Junggar basin. The velocities underlying Moho are 7.7 to 7.8km/s between the Tianshan and the Junggar basin, and 7.9 to 8.0km/s below the Altay Mountains and eastern margin of the Tarim basin. The southern half of the profile, including the eastern Tianshan Mountains and eastern margin of the Tarim basin, shows low P-wave velocities and sigma = 0.25 to a depth of 30km, which suggests a quartz-rich, granitic upper. crustal composition. The northern half of the profile below the Altay Mountains and Junggar Accretional Belt has a higher Poisson's ratio of sigma = 0.26 similar to 0.27 to a depth of 30km, indicative of an intermediate crustal composition. The entire profile is underlain by a 15 to 30km thick high-velocity (6.9 similar to 7.0km/s; a = 0.26 similar to 0.28) lower crustal layer that we interpret to have a bulk composition of mafic granulite. At the southern end of the profile a 5km-thick mid-crustal low-velocity layer (V-P = 5.9km/s, alpha = 0.25) underlies the Tianshan and the region to the south, and may be indicative of granitic intrusive in Late Paleozoic.

A preliminary investigation of Cr geochemistry in serpentinous sediment completed for a multiple-aquifer ground-water monitoring well (Willow core of Santa Clara County, CA) determined sediment at depths >225 meters contains Cr concentrations ranging from 195 to 1155 mg/kg. Serpentinous sediment from this site is a potential source of non-anthropogenic Cr contamination. Chromium-bearing minerals such as Cr-spinel appear to be the main source of Cr in the sediment; however, Cr-bearing silicates and clay minerals are additional Cr sources. Aqueous Cr concentrations in the sediment are <4.6 mg/L; however, the valence of Cr was not identified in the solutions or in the sediment. Although there is no indication of Cr(VI) contamination derived from the serpentinous sediment, elevated Cr concentrations in the sediment, the observed ‘dissolution’ textures of the Cr-bearing minerals, the estimated redox environment, and water chemistry indicate the formation of Cr(VI) is potentially favorable.

We present a new crustal section across northwest China based on a seismic refraction profile and geologic mapping. The 1100‐km‐long section crosses the southern margin of the Chinese Altai Mountains, Junggar Accretional Belt and eastern Junggar basin, easternmost Tianshan Mountains, and easternmost Tarim basin. The crustal velocity structure and Poisson's ratio (σ), which provide a constraint on crustal composition, were determined fromPandSwave data. Despite the complex geology, the crustal thickness along the entire profile is nearly uniform at 50 km. The thickest crust (56 km) occurs at the northern end of the profile beneath the Altai Mountains and the thinnest (46 km) crust is beneath the Junggar basin. Beneath surficial sediments, the crust is found to have three layers withPwave velocities (Vp) of 6.0–6.3, 6.3–6.6, and 6.9–7.0 km/s, respectively. The southern half of the profile, including the eastern Tianshan Mountains and eastern margin of the Tarim basin, shows lowPwave velocities and σ = 0.25 to a depth of 30 km, which suggests a quartz‐rich, granitic upper crustal composition. The northern half of the profile below the Altai Mountains and Junggar Accretional Belt has a higher Poisson's ratio of σ = 0.26–0.27 to a depth of 30 km, indicative of an intermediate crustal composition. The entire 1100‐km‐long profile is underlain by a 15–30 km thick high velocity (6.9–7.0 km/s; σ = 0.26–0.28) lower‐crustal layer that we interpret to have a bulk composition of mafic granulite. At the southern end of the profile, a 5‐km‐thick midcrustal low‐velocity layer (Vp= 5.9 km/s,σ= 0.25) underlies the Tianshan and the region to the south, and may be indicative of a near‐horizontal detachment interface.Pnvelocities are ∼7.7–7.8 km/s between the Tianshan and the Junggar basin, and ∼7.9–8.0 km/s below the Altai Mountains and eastern margin of the Tarim basin. We interpret the consistent three‐layer stratification of the crust to indicate that the crust has undergone partial melting and differentiation after Paleozoic terrane accretion. The thickness (50 km) of the crust appears to be related to compression resulting from the Indo‐Asian collision.

The geochemical composition of the Earth's upper mantle1,2,3 is thought to reflect 4.5 billion years of melt extraction, as well as the recycling of crustal materials. The fractionation of rhenium and osmium during partial melting in the upper mantle makes the Re–Os isotopic system well suited for tracing the extraction of melt and recycling of the resulting mid-ocean-ridge basalt3. Here we report osmium isotope compositions of more than 700 osmium-rich platinum-group element alloys derived from the upper mantle. The osmium isotopic data form a wide, essentially gaussian distribution, demonstrating that, with respect to Re–Os isotope systematics, the upper mantle is extremely heterogeneous. As depleted and enriched domains can apparently remain unequilibrated on a timescale of billions of years, effective equilibration seems to require high degrees of partial melting, such as occur under mid-ocean ridges or in back-arc settings, where percolating melts enhance the mobility of both osmium and rhenium. We infer that the gaussian shape of the osmium isotope distribution is the signature of a random mixing process between depleted and enriched domains, resulting from a ‘plum pudding’ distribution in the upper mantle, rather than from individual melt depletion events.

Reference EPFL-CONF-177134View record in Web of Science Record created on 2012-05-15, modified on 2016-08-09

Reliable data regarding the seismic structure of China and India significantly improve the determination of seismic source parameters for the purposes of monitoring nuclear test explosions in those countries. To provide some of this information, we present seismic cross-sections for China and India and new maps of S-wave attenuation in Eurasia. We present a new deep crustal section across northwest China and the northeastern Tibetan Plateau based on seismic refraction experiments and geologic mapping. The crustal velocity structure and Poisson's ratio (σ) for the 2,700-km-long transect, which provide a constraint on crustal composition, were determined from P- and S- wave data. The crustal thickness along the profile was determined, and the crust was found to have three layers with P-wave velocities (Vp) of 6.0-6.3 km/s, 6.3-6.6 km/s, and 6.9-7.0 km/s, respectively. We interpret the consistent three-layer stratification of the crust to indicate that the crust has undergone partial melting and differentiation after Paleozoic terrane accretion. Pn velocities are ~7.7 to 7.8 km/s. We likewise present deep structural details of central India using reprocessed seismic data. A shallow mid-crust and a thick high-velocity lower crust are found to be characteristic features of the region. The study area exhibits high mantle heat flow and shallow lithospheric thickness compared with other cratons and mobile belts of the Indian shield. The geophysical anomalies of the region are thought to be in response to mantle plume and other tectonic activities during the Neoproterozoic-Phanerozoic period. Some of the magmatic and tectonic events of this region have been correlated to global tectonics and episodes of supercontinent formation during the Neoproterozoic. Finally, we compare maps of the thermal structure of the continental lithosphere with the seismic attenuation of shear waves (QS) determined from surface wave amplitudes. The thermal structure is taken from a recent study (Artemieva and Mooney, 2001) that utilizes thermal parameters (radiogenic heat production and conductivity). QS values are taken from the study of Billien et al. (2001) that used the Trampert and Woodhouse (2001) data set. We find that maps of QS correlate more closely with lithospheric temperature than any other physical parameter, such as P- or S-wave velocities. We compare values of QS and temperature at depths of 50 km and 100 km in the continental lithosphere. The correlation coefficient between temperature and seismic parameters is low and ranges from 0.2 to 0.4.